1. Field of the Invention
The present invention relates to a syringe assembly to be used for delivering a therapeutic liquid into a subject or for drawing blood from a subject, and more particularly to a syringe assembly which is capable of withdrawing the hypodermic needle into the syringe barrel after use.
2. Description of the Related Art
A hypodermic syringe is conventionally employed for delivering a therapeutic liquid into a subject or for drawing blood from a subject.
The hypodermic syringe generally comprises a plunger provided at its distal end with a gasket, and a cylindrical barrel provided at its distal end with a needle engaging means.
Recently, it is increasingly desired to assuredly protect therapeutic technicians from being infected with AIDS, hepatitis, and other infections. However, as a matter of fact, a syringe user sometimes have other works to do such as bleed stopping operation upon the withdrawal of a syringe, so that it is often difficult for the syringe user to pay full attention to the syringe that has been withdrawn from a subject. In such a case, the syringe having it needle tip contaminated with blood of a patient is left for a while in order to do other pressing jobs such as bleed stopping operation, and after finishing these pressing jobs the syringe user can do the capping of the needle and discard the syringe along with the capped needle.
However, when the syringe is left on a table after the withdrawal of the needle, or when the syringe falls on the floor, it may become a cause of infection from blood, or create a risk of accidental pricking of fingers due to the exposed needle.
Recently, there has been proposed a syringe provided with a needle-protecting mechanism, as shown in, for example, U.S. Pat. No. 4,978,340. The syringe proposed in this U.S. Patent comprises a syringe body, a plunger adapted to be retracted into the syringe body, and a needle carrier for engaging a needle, which is detachably mounted through thread-engagement on the distal end of the syringe body. When the plunger is rotated in the direction to release the engagement between the syringe body and the needle carrier, the rotation is transmitted to the needle carrier thereby allowing the needle carrier to be detached together with the needle from the syringe body. Accordingly, it is possible to retract the needle together with the plunger into the syringe body by sliding the plunger rearward.
Since the needle after use can be easily retracted into the syringe body in this manner, the safety of the syringe is assured.
In the above syringe, the engagement between the needle carrier and the plunger is achieved by the engagement between a hollow portion formed on the proximal end of the needle carrier as well as multiple splines formed on the inner surface of the hollow portion and a projected portion formed at the tip portion of the plunger as well as multiple splines conforming to the first mentioned multiple splines and formed on the outer circumference of the projected portion. Thus, the engagement between both of the multiple splines is achieved by inserting the projected portion of the plunger into the hollow portion of the needle carrier, and when the plunger is retracted, the needle carrier is also retracted. Further, since the multiple splines of the needle carrier and plunger are engaged together, torque of the plunger is directly transmitted to the needle carrier.
However, when the plunger is accidentally rotated in a direction which is reverse to the direction of releasing the needle carrier from the syringe body, i.e. a direction to mount the needle carrier on the syringe body (a direction which promotes the thread-engagement of the needle carrier to the syringe body), the torque resulting from this rotation is transmitted through the splines to the needle carrier. As a result, the engagement strength between the needle carrier and the syringe body becomes so large that it is sometimes becomes very difficult to disengage the needle carrier from the syringe body even if the plunger is rotated in the direction to release the engagement between the needle carrier and the syringe body.
Further, since the engagement in axial the direction between the needle carrier and the plunger is achieved through the engagement between the hollow portion of the needle carrier and the projected portion of the plunger, if the plunger is strongly pressed into the syringe body, the plunger may possibly be engaged excessively with the needle carrier, so that it becomes very difficult to move the plunger rearward in an attempt to introduce a liquid medication into the syringe. If, at this occasion, the plunger is happened to be rotated in any direction, the detachment of the needle carrier from the syringe body may be resulted in its rotation in one direction, or otherwise an excessive engagement between the needle carrier and the syringe will be resulted in its rotation in the other direction.